The present invention relates to a reception volume limiting circuit and, more particularly, to a reception volume limiting circuit for limiting the gain of a speech signal received by a mobile telephone set or the like.
As shown in FIG. 5, a conventional reception volume limiting circuit for a telephone set is constituted by a descramble circuit 51 for decoding reception speech signal data, a D/A conversion section 52 for converting the decoded data into an analog signal, a volume control section 53 for voltage-controlling the level of the analog signal, a detector 54 for generating a speech control voltage from the decoded reception speech signal data, and an amplifier 55 (e.g., Japanese Patent Laid-Open No. 62-146032).
A reception volume limiting operation is performed as follows. In a normal descramble (privacy decoding) operation, no volume limiting operation is performed. When a descramble operation cannot be executed, or an abnormal descramble operation is performed, e.g., privacy codes on the transmitting and receiving sides differ from each other, a digital signal is input from the descramble circuit 51 to the detector 54. If speech signals having levels higher than a predetermined value are frequently detected within a predetermined period of time, it is determined that high-level noise is generated. As a result, a control signal Vc is output from the detector 54. The reception volume is limited by the volume control section 53 in accordance with this control signal Vc.
FIG. 6 shows a multi-functional telephone set which detects the voltage of power supplied from an electronic switching apparatus, and limits the volume of a speaker when the voltage becomes lower than a predetermined voltage (e.g., Japanese Patent Laid-Open No. 2-119359).
A reception volume limiting operation is performed as follows. When a cable 62 is extended, a decrease in a power supply voltage applied from an electronic switching apparatus 61 via a pulse transformer 63 is detected by a voltage detector 65. If it is determined that the decrease in voltage causes a trouble in supply of power from a DC/DC converter 64, the volume of a speaker amplifier 67 is limited by a volume limit signal 66. With this operation, the power consumption is limited to prevent a decrease in power supply voltage due to extension of the cable 62, thereby ensuring stable supply of power from the DC/DC converter 64.
The above conventional reception volume limiting circuits perform reception volume limiting operations mainly for the following purposes. The purpose of the former circuit is to reduce an unpleasant sound which is temporarily generated. The purpose of the latter circuit is to ensure stable application of a power supply voltage when a decrease in supply voltage occurs. If, therefore, an excessive reception volume signal is input abruptly or continuously during a normal reception speech signal hearing operation, it is difficult to effectively prevent the listener from having difficulty in hearing.
In addition, a conventional portable telephone set or the like is designed to passively restrict the output sound pressure, within a sound pressure level range in which no difficulty in hearing is caused, on the basis of the set power supply voltage of a receive amplifier or the electroacoustic conversion efficiency of a receiver used. In practice, however, the speech peak level often exceeds the above sound pressure level range owing to the influences of ambient noise and the like. According to the above passive limiting circuit, since the electroacoustic characteristics of the receive amplifier or the receiver exhibit a nonlinear region near the upper limit level, a considerable deterioration in sound quality occurs.